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Question Answer (2 Marks)

Question 512 Marks
What is the force of gravity on a body of mass $150\ kg$ lying on the surface of the earth?
(Mass of earth $=6 \times 10^{24} kg$; Radius of earth $=6.4 \times 10^6 m ; G =6.7 \times 10^{-11} Nm ^2 / kg ^2$ )
Answer
Force due to gravity, $\text{F}=\text{G}\times\frac{\text{m}\times\text{M}}{\text{d}^2}$
$\text{F}=6.7\times10^{-11}\times\frac{6\times10^{24}\times150}{(6.4\times10^6)^2}$
$\text{F}=1472\text{N}$
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Question 522 Marks
Why does a sharp knife cut objects more effectively than a blunt knife?
Answer
A sharp knife cuts objects easily because due to its very thin edge, the force of our hand falls on a very small area of the object producing large pressure.
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Question 532 Marks
Explain why, a nail has a pointed tip.
Answer
A nail has a pointed tip, so that when it is hammered, the force of hammer is transferred to a very small area of wood creating a large pressure which pushes the nail into the wood.
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Question 542 Marks
Write down an equation that defines density.
Answer
Density of substance $=\frac{\text{mass of substance}}{\text{volume of substance}}$
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Question 552 Marks
Give reason for the following:
The force of gravitation between two cricket balls is extremely small but that between a cricket ball and the earth is extremely large.
Answer
The force of gravitation between two bodies is directly proportional to the product of their masses.
$\text{F}\propto\text{m}\times\text{M}$
Since the mass of cricket balls is very small as compared to that of the earth, so the force of gravitation between two cricket balls is extremely small while that between a ball and the earth is extremely large.
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Question 562 Marks
A coin and a piece of paper are dropped simultaneously from the same height. Which of the two will touch the ground first? What will happen if the coin and the piece of paper are dropped in vacuum? Give reasons for your answer.
Answer
The coin reaches the ground first as compared to the piece of paper because it experiences lesser resistance from air than that felt by paper.
If the coin and the piece of paper are dropped in vacuum, both of them will touch the ground at the same time.
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Question 572 Marks
An object weighs 500 grams in air. This object is then fully immersed in water. State whether it will weigh less in water or more in water. Give reason for your answer.
Answer
The object will weigh less in water because an upward force (buoyant force) equal to the weight of water displaced acts on the object when immersed in water which reduces its weight apparently.
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Question 582 Marks
A floating boat displaces water weighing 6000 newtons.
  1. What is the buoyant force on the boat?
  2. What is the weight of the boat?
Answer
Weight of water displaced by boat = 6000N.
  1. Buoyant force = 6000N, as the weight of water displaced is equal to buoyant force.
  2. Weight of a floating object = Weight of water displaced by it = 6000N.
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Question 592 Marks
Explain why, a wide steel belt is provided over the wheels of an army tank.



A wide steel belt is provided over the wheels of an army tank.
Answer
A wide steel belt is provided over the wheels of an army tank so that they exert less pressure on the ground and do not sink into it.
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Question Answer (2 Marks) - Page 2 - Science STD 9 Questions - Vidyadip